High-frequency dielectric-heating furnace



Oct. 11, 1960 A. H. M. HIJMANS HIGH-FREQUENCY DIELECTRIC-HEATING FURNACE Filed Jan. 9, 1959- FiG. 1

INVENT ANTONIUS HENRICUS MARIA HIJfiANS HIGH-FREQUENCY DIELECTRIC-HEATING FURNACE Antonius Henricus Maria Hijmans, Emmasingel, Eindhoven, Netherlands, assignor to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed Jan. 9, 1959, Ser. No. 785,831

Claims priority, application Netherlands Feb. 1, 1958 8 Claims. (Cl. 21910.77)

The invention relates to a high-frequency, dielectric heating furnace circuit arrangement comprising a fedback electron-tube oscillator having a resonant circuit included in its anode circuit. The circuit arrangement further comprises a load circuit including a charging capacitor. The anode-circuit coil is coupled inductively with the load-circuit coil. The high-frequency furnace according to the invention is particularly important for high power performances, for example of 1 kw. and more at very high frequencies, for example in the frequency range from 40 to 200 mc./ s.

The invention has for its object to provide a high-frequency furnace of the aforesaid kind, which is particularly advantageous in practice and in which, in a simple manner, an adaptation of the load to the oscillator tube is obtained over a very wide range, which is desired, for example, with dielectric heating of work pieces of highly different kind and size.

The high-frequency furnace according to the invention has the feature that the relatively stationary anode-circuit coil and load-circuit coil, which are structurally integral with each other, are continuously variable to ensure adaptation to load. Furthermore, each of these variable coils is provided with adjusting means which are rigidly secured to a common control-member. When the common control-member is actuated, vary simultaneously the values of the inductance of the anode-circuit coil and of the 1oad eircuit coil in opposite senses.

According to a further aspect of the invention the anode-circuit coil and the load-circuit coil each consist of a cylindrically curved, flat strip of conductive material.

The strips are arranged side by side in an axial direction and fixed in place, the common control-member shaped in the form of a rotatable shaft being arranged in the axial direction of the strips. Each of the coil adjusting means seated rigidly on the control-shaft is formed by a contact arm, which moves, upon rotation of the common control-shaft, along the inner wall of each of the.

anode-circuit coil and the load circuit coil cylindrically curved strip.

Apart from the rugged mechanical structure, the embodiment described above for high power processes at high frequencies has important advantages. In the first place, owing to the large outer surface of the coils constructed in the form described skin effect cannot adversely affect the output. Furthermore, due to the large cooling surface of these coils forced wateror air-cooling can be dispensed with.

shown in Fig. 1 and suitable for a performance of 4- kw. at frequencies of more than 40 mc./s., the high-frequency United States Patent energy required for heating is derived from a triode oscillator, having a triode 1. The anode circuit of the oscillator, constructed in the form of a Colpitts oscillator, includes a circuit coil 2, a circuit capacitor 3 and a capacitative potentiometer included between the anode and the control-grid. The potentiometer is formed by the anodeconnected parallel combination of a stationary capacitor 4 and an adjustable capacitor 4', connected in series with a fixed capacitor 5. The tapping point of the potentiometer formed by the capacitors 4, 4', 5 is connected to the grounded cathode of the tube 1. The end of the potentiometer capacitor 5, remote from the tapping is connected via a grid capacitor 6 to the control-grid of the tube 1. The control grid of the tube 1 is connected to the cathode via the series combination of a high-frequency choke 7 and a resistor 9, shunted by a capacitor 8. The supply voltage of the triode 1 is obtained from a direct voltage source 10, which is connected via a supply choke 11 and the anode circuit coil 2 to the anode of the triode 1.

In the high-frequency furnace for very high frequencies, as shown, the capacitors 3, 4, 5 are formed by the tube capacities and the additional stray capacities of the arrangement.

The high-frequency energy produced during the oscillation of the arrangement is supplied to a load circuit, constituted by a charging capacitor 12, which includes a work piece 13, and a load circuit coil 14. The load circuit coil 14 is coupled inductively with the anode circuit coil 2.

In order to adapted the load to the generator under varying operational circumstances it is required with the high-frequency furnace shown, having a tuned load circuit 12, 14 and a tuned anode circuit, consisting of the circuit coil 2 and the total circuit capacity formed by the capacitors 3, 4, 4', 5, that the frequencies of the anode circuit 2, 3, 4, 4, 5 and of the load circuit 12, 14 should be adjusted relatively to each other. This is due to the fact that in accordance with the kind and size of the work piece 13 to be heated the tuning frequency of the load circuit 12, 14 may deviate strongly from the tuning frequency of the anode circuit 2, 3, 4, 4', 5 and hence from the oscillations produced by the high-frequency furnace circuit arrangement. This may result in an inadequate energy transfer to the work piece 13, to be heated. Thus, for example, when heating a rod-shaped work piece, the distance between the plates of the charging capacitor 12 is large, so that the capacity of the charging capacitor 12 is large, so that the capacity of the charging capacitor 12 is small and the tuning frequency of the load circuit 12, 14 will be comparatively high. If a fiat work piece is heated, however, the distance between the plates of the charging capacitor 12 is small, so that the capacity of said charging capacitor is large and the tuning frequency of the load circuit 12, 14 has a low value.

In order to obtain a simple adaptation control in a large range of adaptations, the relatively stationary anode circuit coil 2 and load circuit coil 14 are continuously variable. Each of the variable coils 2, 14 are provided with adjusting means 16 and 17 respectively secured rigidly to a common control-member 15. The adjusting means 16 and 17 vary in opposite senses simultaneously the values of the inductances of the anode circuit coil 2 and the load circuit coil 14, when the common controlmember 15 is actuated. The relatively fixed anode circuit coil 2 and the load circuit coil 14 are structurally integral with each other.

The adaptation control, which includes the relative frequency adjustment of the anode circuit 2, 3, 4, 4, 5 and the load circuit 12, 14 is obtained in the high-frequency furnace shown by means of the common controlmember 15, which varies in opposite senses the values of the inductances of the anode circuit 2 and the load circuit 14. If it is assumed that by introducing a work hereinafter. maximum inductances of the anode circuit coil 2 are L and pL and the minimum and the maximum inductances of the load circuit coil 14 are also L and pL and .the value of the total anode circuit capacity 3, 4, 4', 5 is vThis is achieved by reducing simultaneously the value of theinductance of the anode circuit coil 2 and increasing that of the load circuit coil 14 with the aid of the common control-member 15. Conversely, if the tuning frequency of the load circuit 12, 14 is lower by the introduction of a work piece into the charging capacitor 12, than that of the anode circuit 2, 3, 4, 4', 5, the tuning fre- .quency of the anode circuit 2, 3, 4, 4, 5 will be reduced and that of the load circuit 12, 14 will be increased with the aid of the common control-member 15.

An accurate load adaptation of the work piece 13 to the high-frequency furnace is obtained by detuning the anode circuit 2, 3, 4, 4', 5 and the load circuit 12, 14 relatively to each other to some extent with the aid of the common control-member 15 in accordance with the value of the load. This is due to the fact that the value of the load formed by the work piece 13, viewed from the oscillator tube 1, varies with the relative detuning between the load circuit 12, 14 and the anode circuit 2, 3, 4, 4', 5. Thus, with the aid of the common control- .member 15 opposite variations of the anode circuit coil .2 and'the load circuit coil 14 bring about the adaptation of the work piece 13 to the generator.

The adaptation range of the high-frequency furnace as shown is very large, as will be explained more fully It is assumed that the minimum and the C If, in this device, the anode circuit coil 2 has its minimum value L, the load circuit coil 14 has its maximum value pL and the charging capacitor 12 has a value of 1/p.C in order to ensure equal tuning frequencies of the load circuit 12, 14 and of the anode circuit 2, 3, 4, 4, 5. Conversely, with a maximum value pL of the anode circuit coil 2 and a minimum value of L of the load circuit coil 14, the charging capacitor 12 has a value of pC In accordance with the size of the work piece 13 to be heated, the charging capacitor 12 may be varied between example more than 40 mc./ s. has, apart from the simple I adaptation control and the very large adaptation range, the important advantage that an optimum transfer of energy to the work piece is obtained throughout the adaptation range. The applicant has found, in practice, that in the most highly differing operational conditions there is no adverse effect on the working efliciency owing to the coupling frequencies of the coupled circuits 2, 3, 4, 4', and 12, 14. Nor is there an abrupt leap of the oscillator frequency to a parasitic frequency, which phenomenon is due to the inductance of the supply conductors and further parasitic impedances in the arrangement. It is these factors which can no longer be neglected in the case of high frequencies.

It is advantageous in this respect that the adaptation control described does not require additional elements and that a separation capacitor to obtain direct-current separation of the anode circuit and the load circuit rnay be dispensed with. Such a separation capacitor, together with the parasitic impedances in the arrangement, appears to give rise to parasitic oscillations affecting adversely the efficiency under highly dilfering operational conditions.

Fig. 2 is a perspective view of a structural embodiment of the anode circuit coil 2 and the load circuit coil 14, constructed as a mechanical unit. Corresponding elements of Figs. 1 and 2 are designated by the same reference numerals.

In the mechanical structure shown the anode circuit coil and the load circuit coil are constituted by cylindrically curved copper tapes 2 and 14 respectively, which are stationary in an axial direction side by side. In order to illustrate the construction employed, part of the coil 14 of cylindrical tape is shown broken away, for the sake of clarity.

The two coils 2 and 14 are relatively secured in place via insulators 18 and secured rigidly to a frame 20 with the aid of insulators 19. It is evident from the figure that the anode circuit coil 2 and the load circuit coil 14, constituted by cylindrically curved tapes, have a large outer surface, so that the efficiency is not affected by skin effect. The large cooling surfaces of the coils 2 and 14 thus formed renders unnecessary forced airor water-cooling, even in the case of high power processes.

The common control member is provided in the axial direction of the cylindrically curved tapes 2 and 14 in the form of a rotatable shaft 15 of insulating material, which can be rotated for adaptation control with the aid of a hand wheel (not shown in the figure). On either side of the cylindrically curved, flat tapes 2 and 14 the rotatable shaft 15 is journalled in mounting pieces 21 and 22 of insulating material, each of which is secured rigidly with the aid of screws, to the two ends 27, 28 and 29, 30 respectively of the cylindrically curved, flat tapes 2 and 14 respectively.

On the control-shaft 15 are seated, as adjusting members of the coils 2 and 14, two plate-shaped copper contact arms 16 and 17, which extend in a radial direction of the anode circuit coil 2 and the load circuit coil 14. The contact arms 16 and 17 are seated rigidly on the control-shaft 15 with the aid of cylindrical fastening sleeves 23 and 24. The fastening sleeves 23 and 24 of the contact arms 16 and 17 respectively constitute contact paths for stationary, plate-shaped, copper contact arms 25 and 26, secured to the mounting pieces 21 and 22. The contact arms 25 and 26 extend in a radial direction of the anode circuit coil 2 and the load circuit coil 14. The connecting terminals of the anode circuit coil 2 are constituted by the end 31 of the stationary contact plate 25, secured to the mounting piece 21 and by the end 28 of the cylindrical tape. The ends 31 and 28 are secured via plate-shaped conductors 36 to the anode and the control-grid of the tube 1. The charging capacitor 12 is positioned and connected between the end 32 of the stationary contact plate 26, secured to the mounting piece 22, and the end 29 of the load circuit coil 14, via plate-shaped conductors 37.

In order to obtain an excellent electrical contact the movable contact plates 16, 17 establish a contact substantially throughout the width along a generatrix of the inner walls of the anode circuit coil 2 and the load circuit coil 14 by means of contact pins 33 and 34 respectively, subjected to spring-pressure. The stationary contact plates 5 and 26 establish a contact throughout the width along a generatrix of the contact paths 23 and 24 respectively with the aid of contact pins subjected to spring pressure. The contact pins associated with the stationary contact plate 25 are designated by 35 and the contact pins associated with the stationary contact plate 26 are not shown in the figure.

If in the construction described above the common shaft 15 is rotated, the movable contact arms 16 and 17 move along the inner walls of the anode circuit coil 2 and the load circuit coil 14, so that, as is evident from figure, simultaneously the values of the inductances of the anode circuit coil 2 and the load circuit coil 14 are varied in opposite sense. In the extreme positions of the contact arms 16 and 17 the minimum inductances of the anode circuit coil 2 and of the load circuit coil 14 are reduced in the construction shown to a very low value, which is essential to otbain a large adaptation range. This is due to the fact, indicated above, that the adaptation range is proportional to the square of the ratio between the minimum and the maximum inductances of the anode circuit coil 2 and the load circuit coil 14.

By using the arrangement shown in a mechanically simple and rugged construction of the high-frequency furnace, the control and the electrical properties thereof are particularly advantageous for high power processes at very high frequencies.

By way of example, data is indicated below for a practical embodiment of a high-frequency furnace as described above, suitable for powers of 4 kw. in the frequency range between 40 and 200 ms./ s.

The triode 1 is of the type TBL 6/6000.

The diameter of each of the cylindrically curved tapes 2 and 14 is 220 millimeters.

The width of each of the tapes 2, 14, measured in an axial direction, is 80 mms.

Relative distance between the tapes 2, 14: 20 mms.

Ratio between minimum value and maximum value of the charging capacitor 12: about 1:30.

In the embodiments of the anode circuit coil 2 and the load circuit coil 14 shown in Fig. 2 these coils are of identical structure, which has the mechanical advantage that a minimum of different parts will be sufficient. However, this is not an imperative requirement; in certain conditions it may, for example, be advantageous to vary the size of the load circuit coil 14 from that of the anode circuit coil 2.

It should finally be noted that, with respect to the simple adjusting possibility in the high-frequency furnace shown, this furnaceis particularly suitable for automatic control.

What is claimed is:

1. A high frequency furnace circuit arrangement comprising an oscillator circuit including an electron tube having an output electrode and a resonant circuit having a first inductive coil connected to said output electrode,

a load circuit comprising heating means and a second inductive coil connected to said heating means, said secnd coil being inductively coupled to said first coil, said first and second coils being structurally integral with each other, and adjusting means for continuously simultaneously varying the inductance of each of said first and second coils in a sense opposite to that of the other.

2. A high frequency furnace circuit arrangement comprising an oscillator circuit including an electron tube having an output electrode and a resonant circuit having a first inductive coil connected to said output electrode, said first coil comprising a substantially cylindrically curved flat strip of electrically conductive material, a load circuit comprising heating means and a second inductive coil connected to said heating means, said second coil comprising a substantially cylindrically curved flat strip of electrically conductive material positioned coaxially with and adjacent to said first tape and being inductively coupled to the said first coil, said first and second coils being structurally integral with each other, and adjusting means for continuously simultaneously varying the inductance of each of said first and second coils in a sense opposite to that of the other.

3. A high frequency furnace circuit arrangement comprising an oscillator circuit including an electron tube having an output electrode and a resonant circuit having .a first inductive coil connected to said output electrode, said first coil comprising a substantially cylindrically curved fiat strip of electrically conductive material, a load circuit comprising heating means and a second inductive coil connected to said heating means, said second coil comprising a substantially cylindrically curved flat strip of electrically conductive material positioned coaxially with and adjacent to said first tape and being inductively coupled to the said first coil, said first and second coils being structurally integral with each other, and adjusting means for continuously simultaneously varying the inductance of each of said first and second coils in a sense opposite to that of the other, said adjusting means comprising a common control member adapted to actuate the said adjusting means upon actuation thereof.

4. A high frequency furnace circuit arrangement comprising an oscillator circuit including an electron tube having an output electrode and a resonant circuit having a first inductive coil connected to said output electrode, said first coil comprising a substantially cylindrically curved fiat strip of electrically conductive material, a load circuit comprising heating means and a second inductive coil connected to said heating means, said second coil comprising a substantially cylindrically curved fiat strip of electrically conductive material positioned coax-ially with and adjacent to said first tape and being inductively coupled to the said first coil, said first and second coils being structurally integral with each other, and adjusting means for continuously simultaneously varying the inductance of each of said first and second coils in a sense opposite to that of the other, said adjusting means comprising a common control shaft rotatably mounted on the axis of said coils and means for varying the inductance of each of said first and second coils in a sense opposite to that of the other rigidly afiixed to said shaft in a manner whereby said inductance varying means rotate with the said shaft and said inductance is so varied upon rotation of the said shaft.

5. A high frequency furnace circuit arrangement comprising an oscillator circuit including an electron tube having an output electrode and a resonant circuit having a first inductive coil connected to said output electrode, said first coil comprising a substantially cylindrically curved fiat strip of electrically conductive material, a load circuit comprising heating means and a second inductive coil connected to said heating means, said second coil comprising a substantially cylindrically curved flat strip of electrically conductive material positioned coaxially with and adjacent to said first tape and being inductively coupled to the said first coil, said first and second coils being structurally integral with each other, and adjusting means for continuously simultaneously varying the inductance of each of said first and second coils in a sense opposite to that of the other, said adjusting means comprising a common control shaft rotatably mounted on the axis of said coils and contact arms contacting at one end thereof the inner surface of each of said coils and rigidly affixed at the other end thereof to said shaft in a manner whereby said contact arms rotate with the said shaft and the inductance of each of said first and second coils is varied in a sense opposite to that of the other upon rotation of the said shaft.

6. A high frequency furnace circuit arrangement com prising an oscillator circuit including an electron tube having an output electrode and a resonant circuit having a first inductive coil connected to said output electrode, said first coil comprising a substantially cylindrically curved flat strip of electrically conductive material, a load circuit comprising heating means and a second induct-ive coil connected to said heating means, said second coil comprising a substantially cylindrically curved fiat strip of electrically conductive material positioned coaxially with and adjacent to said first tape and being inductively coupled to the said first coil, said first and second coils being structurally integral with each other, and adjusting means for continuously simultaneously varying the inductance of each of said first and second coils in a sense opposite to that of the other, said adiust ing means comprising a common control shaft rotatably mounted on the axis of said coils and a pair of contact arms, each of said contact arms comprising an electrically conductive plate contacting at one end thereof the inner surface of one of said coils and rigidly afiixed at the other end thereof to said shaft in a radially extending manner whereby said contact arms rotate with the said shaft and the inductance of each of said first and second coils is varied in a sense opposite to that of the other upon rotation of the said shaft.

7. A high frequency furnace circuit arrangement comprising an oscillator circuit including an electron tube coil connected to said heating means, said second coil comprising a substantially cylindrically curved flat strip of electrically conductive material positioned coaxially with and adjacent to said first tape and being inductively coupled to the said first coil, said first and second coils being structurally integral with each other, and adjust- ,ing means for continuously simultaneously varying the inductance of each of said first and second coils in a sense opposite to that of the other, said adjusting means comprising a common control shaft rotatably mounted on and rigidly affixed by a cylindrical fastening sleeve at the other end thereof to said shaft in a radially extending manner, and first and second stationary contact arms each comprising an electrically conductive plate contacting at one end thereof a terminal edge of one of said coils and contacting at the other end thereof the fastening sleeve of one of said coils in a manner whereby said firstmentioned contact arms rotate with the said shaft and the inductance of each of said first and second coils is varied in a sense opposite to that of the other upon rotation of the said shaft.

8. A high frequency furnace circuit arrangement comprising an oscillator circuit including an electron tube having an output electrode and a resonant circuit having a first inductive coil connected to said output electrode, said first coil comprising a substantially cylindrically curved flat strip of electrically conductive material, a load circuit comprising heating means and a second inductive coil connected to said heating means,'said second coil comprising a susbtantially cylindrically curved fiat strip of electrically conductive material positioned coaxially with and adjacent to said first tape and being inductively coupled to the said first coil, said first and second coils being structurally integral with each other, and adjusting means for continuously simultaneously varying the inductance of each of said first and second coils in a sense opposite to that of the other, said adjusting means comprising a common control shaft rotatably mounted on the axis of said coils, first and second contact arms each comprising an electrically conductive plate contacting at one end thereof the inner surface of one of said coils and rigidly afiixed by a cylindrical fastening sleeve at the other end thereof to said shaft in a radially extending manner, contacting means at said one end of each said first and second contact arms comprising a plurality of contact pins and spring means applying pressure to said contact pins, first and second stationary contact arms-each comprising an electrically conductive plate contacting at one end thereof a terminal edge of one of said coils and contacting at the other end thereof the fastening sleeve of one of said coils in a manner whereby said first-mentioned contact arms rotate with the said shaft and the inductance of each of said first and second coils'is varied in a sense opposite to that of the other upon rotation of the said shaft, and contacting means at said other end of each of said first and second stationary contact arms comprising a plurality of contact pins and spring means applying pressure to said contact pins.

References Cited in the file of this patent UNITED STATES PATENTS 

